1. Field of the Invention
The present invention relates to an optical communication node and an optical network and, in particular, it relates to prevention of erroneous connection of optical communication nodes and optical networks having route switching functions in a wavelength division multiplex (WDM) network system.
2. Description of the Related Art
In recent years, as communication on the Internet, image transmission and the like becomes widespread, an WDM system that is suitable for large capacity and high speed data transmission using optical signals has been introduced. First, the WDM system has been introduced to long distance networks where the WDM system had much economic merit. At present, as its installation cost has been reduced due to the maturity of the technology, the WDM system is being introduced also in intracity core rings.
FIG. 1 shows an example of a metro ring network.
Conventionally, intra-company LANs between buildings and a metro-oriented system between suburb systems have been oriented to a ring network. As shown in FIG. 1, a metro domain is divided into one domain that is comprised of metro access networks A and B close to the subscriber side, such as inter-building connection in urban districts, and the other domain that is comprised of a metro core network (that is also referred to as a metro interoffice, or a metro IOF) that interconnects the metro access networks in the urban districts or is connected to a long distance network.
Today, WDM is applied also to the metro access network, wherein a passive OADM (optical add drop multiplexing) method is the mainstream method.
FIGS. 2A and 2B show an example of the metro access network using the passive OADM method. FIG. 2A shows an example of a two fiber ring network that is comprised of 4 nodes A-D, while FIG. 2B shows an example of a one-way intra-device configuration in each of the nodes A-D.
In FIG. 2B, among WDM signals of 32 wavelengths (λ1-λ32) that are transmitted on a fiber 10 in any one direction, a band signal having wavelengths λ1-λ3 is dropped by a fixed wavelength filter 11 such as a dielectric multilayer filter, and then demultiplexed into light signals of each wavelength λ1-λ3 by an optical branching filter 12 at the next stage. After undergoing processes such as optical amplification and wavelength conversion by corresponding transponders 13-15, each wavelength signal λ1-λ3 is output to an external network device such as a router installed in the node.
On the other hand, a signal that is input from the external network is converted into wavelength signals λ4-λ6 by the corresponding transponders 23-25, respectively. These are multiplexed into a signal of wavelengths λ4-λ6 by an multiplexer 22 and then added to the WDM signal of 32 wavelengths (λ1-λ32) transmitted on the fiber 10 via a fixed wave length filter 21.
As described above, according to the passive OADM method, any light signal of a particular wavelength or wavelength group can be readily added to or dropped from the WDM signal. However, since each node uses dedicated parts such as the fixed wavelength filters 11 and 21 that are adjusted for the corresponding devices, a signal path route (the wavelength or wavelength group to be used) assigned for each node is fixed.
As a result, in network design according to the passive OADM method, it is necessary to determine transmission routes and transmission capacity in advance and dispose filters for the determined conditions. Further, when the transmission routes or the transmission capacity are changed after the service is started, it is necessary to disconnect the network and terminate the service, and then add filters that correspond to new conditions.
In the future, it is expected that optical WDM ring networks will be introduced into the metro core network that is located between the access networks and the long distance networks. As shown in FIG. 1, the metro core network is based on the ring configuration because of the ease of the protection technique and large-scale networks are constructed by connecting the ring configurations in a multistage manner. Also, the metro core network is connected to the rings of the metro access networks such as the passive OADM method as shown in FIG. 2A using unoccupied nodes, or is connected to the long distance networks.
In such a case, when data signals concentrated from the access networks are exchanged between the nodes, or line capacity is increased so as to connect to the long distance networks, for example, a technique for changing and switching line routes becomes important. It is expected that the paths (line routes) will be changed more frequently in the future and the switching will have to be performed automatically for the three reasons described below:                (1) As the network size and demand for the network is increased, the line route setting will be changed more frequently,        (2) In order to provide IP over WDM, a protection feature will be provided at the side of the WDM system. In this case, due to bit-rate independence, it is necessary to perform wavelength switching in an optical layer and complete the switching within a short time (50-100 ms) corresponding to a SONET (Synchronous Optical Network) ring, and        (3) In the future, as a wavelength time sharing service is provided, the line route will be switched more frequently. Also in this case, the switching must be completed within the short time mentioned above.        
In consideration of the above facts, it is necessary to support features for flexibly accommodating disconnection of networks, suspension of service and the like due to the change of transmission routes or transmission capacity of the metro access networks and for performing switching of connection destinations remotely while preventing erroneous connection.
FIG. 3 schematically describes operation of a ring network having path protection features according to an OSPPR (Optical Shared Protection Path Ring) method.
In FIG. 3, one signal is transmitted on one fiber of a two fiber ring in a clockwise direction and the other signal is transmitted on the other fiber in a counterclockwise direction. Here, a group of wavelength signals transmitted on each fiber is divided into work signals and protection signals. For example, in the group of the wavelength signals in the clockwise direction, even wavelengths are assigned to the work signals and odd wavelengths are assigned to the protection signals. On the other hand, in the group of the wavelength signals in the counterclockwise direction, odd wavelengths are assigned to the work signals and even wavelengths are assigned to the protection signals.
In normal communication, a transmitting end node A transmits a signal to a receiving end node D on a work path λx in the clockwise direction shown in a solid line in the figure by using the work signal. The receiving end node D receives the signal by selecting the work path. On the other hand, the corresponding protection path λx in the counterclockwise direction, that is shown as a dotted line in the figure, is idle.
Here, if any line failure, such as a break of the line, occurs in the work path, the transmitting end node A switches the path to the side of the protection path to continue transmission of the signal. After that, the receiving end node D also switches the path to the side of the protection path to continue reception of the signal. Here, it is to be noted that the signal interrupted by the line failure and the like must be recovered within 50-100 ms by the protection action.
In this connection, so as to improve operational efficiency of such configuration, path sharing is typically implemented by providing a signal PCA (protection channel access) path having a lower priority appropriately on the route for protection. In this case, when the failure occurs at the work side, the PCA signal is stopped at the protection side and a protection signal having a higher priority is inserted.
The line route switching occurs frequently and not only when the failure of the line route occurs but also when the line is operated normally due to the wavelength time-sharing service and the like.
In the switching of the line route setting described above, a network management system must manage the switching procedure and the switching timing and provide instructions appropriately but, conventionally, as shown below, there have been problems in that the erroneous connection of the paths might occur when the line route setting was switched, or operation of the instruction system for preventing such erroneous connection might be delayed.
FIGS. 4A and 4B show an example of the erroneous connection that may occur at the time of protection operation.
FIG. 4A shows a normal communication condition. Here, a signal is transmitted in a counterclockwise direction from a transmitting end node D to a receiving end node B via a work path λ1, and a PCA signal having a lower priority is transmitted in a clockwise direction from a transmitting end node A to a receiving end node C via its corresponding protection path λ1.
FIG. 4B shows a case wherein a line failure such as a break of a fiber occurs in the work path λ1 between the node C and the node D. In this case, the line route is switched to the protection route λ1 by the protection operation. As a result, the signal from the transmitting end node D is transmitted to the receiving end node B through the node A in the clockwise direction. The relay node A terminates transmission of the own PCA signal and passes the signal from the transmitting end node D, and the receiving end node B switches the receiving route so as to receive the signal from the protection path λ1.
In the case described above, relative switching timing of the line routes between the nodes A-D, or switching sequence of them will become a problem. When the switching is started from the receiving end node B, after the clockwise route is switched from the through mode to the drop mode at the node B and until the relay node A stops the transmission of the PCA signal and then is switched to the through mode, the PCA signal from the node A is erroneously connected to the receiving end node B and output to an external network.
On the contrary, when the switching is started from the transmitting end node D, the transmitting end node D first switches the line route to the clockwise direction. Then, after the relay node A stops the transmission of the PAC signal and then switches the route to the through mode and until the receiving end node B switches the route in the clockwise direction from the through mode to the drop mode, the signal from the transmitting end node D is erroneously connected to the node C that is the receiving end node of the PCA signal and output to the external network.
FIG. 5 shows an exemplary node configuration for describing the action of the erroneous connection mentioned above more specifically.
First, the node configuration in FIG. 5 will be described briefly. A WDM signal input from an optical fiber 31 in a clockwise route is amplified by an optical preamplifier 37, and then demultiplexed into each wavelength signal (λ1-λn) to be output by an optical branching filter 38. Further, a portion of the input signal is input to an optical supervisory channel (OSC) 33 by an optical branching filter 32.
The optical supervisory channel 33 converts the input signal into an electric signal to give it to a processing/controlling section 34 at the next stage. The processing/controlling section 34 checks communication conditions of the optical fiber 31 in the clockwise route relying on the input signal and the like, and if there is any failure, performs switch control inside the node and the like. Further, a pilot signal and others to be given to the node at the next stage are output via an optical supervisory channel 35 and an optical multiplexing section 36 at the output side.
Next, there will be given a description of the switching action in the node, wherein a work signal λ1 (w) in the clockwise direction that is demultiplexed by the optical branching filter 38 is input to a 2×2 switch 39. If the 2×2 optical switch 39 is set to an add/drop mode, it drops the input work signal λ1 (w) and outputs it to an external network 59 via an optical coupler 45 at the next stage and transponders 47 and 48 in a redundant configuration.
On the other hand, a signal from the external network 59 is input via either one of transponders 51 or 52, which are configured redundantly, and a 1×2 optical switch 50 to the 2×2 switch 39, which, in turn, adds the signal and outputs it as one wave in a WDM signal to the optical fiber 31 via an optical attenuator 40, an optical multiplexer 41, an optical postamplifier 42 and an optical multiplexer 36 at the next stage.
Alternatively, if the 2×2 optical switch 39 is set to a through mode, it passes the input work signal λ1 (w) and outputs it to the optical fiber 31 in the clockwise direction as one wave in the WDM signal via the optical attenuator 40, the optical multiplexer 41, the optical postamplifier 42 and the optical multiplexer 36 at the next stage. Its corresponding protection signal λ1 (p), that is input from an optical fiber 43 in a counterclockwise route, is handled similarly by a 2×2 optical switch 44.
Here, in the lower central part of the figure, it is to be noted that there is also shown an interface with the external network 59 of a direct connection type wherein the 2×2 optical switch is connected to the external network 59 directly, in place of the one of a transponder type wherein an optical signal is once converted into an electric signal and then converted into a predetermined optical signal. In this case, a WDM optical transmitter/receiver is incorporated into network devices out of the ring.
Next, based on the premise of the node configuration described above, the process of the erroneous connection shown in FIG. 4B will be described more specifically. As a result of the line failure occurring between the node C and the node D, if the receiving end node B switches the setting of the 2×2 optical switch 44 from the through mode to the add/drop mode to turn on an optical coupler 46, during the time when the relay node A switches the 2×2 optical switch 44 from the add/drop mode to the through mode, the PCA signal from the node A is output to the external network 59 of the node B.
On the other hand, if the transmitting end node D sets the 2×2 optical switch 44 to the add/drop mode and turns on the 1×2 optical switch 49 to start transmission in the clockwise line route, and then the relay node A switches the 2×2 optical switch 44 from the add/drop mode to the through mode, during the time when the receiving end node B switches the 2×2 optical switch 44 from the through mode to the add/drop mode, the signal from the node D is output to the external network 59 of the node C.
Further, if each node is instructed on switching procedures from the network management side in order to prevent the erroneous connection described above, there is a problem in that a load at the network management side may become excessively large and the time that is necessary for switching the line route in the entire node may be prolonged.